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Register a new userRole of surface roughness in hard x-ray emission from femtosecond laser produced copper plasmas
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Added by
Paramel Pattathil Rajeev
Publication date
2001
fields
Physics
and research's language is
English
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The hard x-ray emission in the energy range of 30-300 keV from copper plasmas produced by 100 fs, 806 nm laser pulses at intensities in the range of 10$^{15}-10^{16}$ W cm$^{-2}$ is investigated. We demonstrate that surface roughness of the targets overrides the role of polarization state in the coupling of light to the plasma. We further show that surface roughness has a significant role in enhancing the x-ray emission in the above mentioned energy range.
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This paper presents a simplified theoretical model for the study of emission from laser produced plasma to better understand the processes and the factors involved in the onset of saturation in plasma emission as well as in increasing emission due to plasma confinement. This model considers that plasma emission is directly proportional to the square of plasma density, its volume and the fraction of laser pulse absorbed through inverse Bremsstrahlung in the pre-formed plasma plume produced by the initial part of the laser. This shows that plasma density and temperature decide the threshold for saturation in emission, which occurs for electron ion collision frequency more than 10E13 Hz, beyond which plasma shielding effects become dominant. Any decrease in plasma sound (expansion) velocity shows drastic enhancement in emission supporting the results obtained by magnetic as well as spatial confinement of laser produced plasma. The temporal evolution of plasma emission in the absence and presence of plasma confinement along with the effect of laser pulse duration are also discussed in the light of this model.
Bright Ar K-shell x-ray with very little background has been generated using an Ar clustering gas jet target irradiated with an 800 mJ, 30 fs ultra-high contrast laser, with the measured flux of 1.1 x 10^4 photons/mrad^2/pulse. This intense x-ray source critically depends on the laser contrast and the laser energy and the optimization of this source with interaction is addressed. Electron driven by laser electric field directly via nonlinear resonant is proved in simulation, resulting in effective electron heating and the enhancement of x-ray emission. The x-ray pulse duration is demonstrated to be only 10 fs, as well as a source size of 20 um, posing great potential application for single-shot ultrafast x-ray imaging.
Betatron x-ray sources from laser-plasma accelerators combine compactness, high peak brightness, femtosecond pulse duration and broadband spectrum. However, when produced with Terawatt class lasers, their energy was so far restricted to a few kilo-electronvolt (keV), limiting the range of possible applications. Here we present a simple method to increase the energy and the flux by an order of magnitude without increasing the laser energy. The orbits of the relativistic electrons emitting the radiation were controlled using density tailored plasmas so that the efficiency of the Betatron source is significantly improved.
Simultaneous measurements of hard X-ray by a Geiger counter and audible sound (10 Hz-20kHz) by a microphone from a thin water film in air were carried out under intense single and double pulse irradiations of femtosecond laser (35 fs, 800 nm, 1 kHz). Emission profiles of X-ray and sound under the single pulse irradiation by changing the water film position along the laser incident direction (Z-axis) show the same peak positions with a broader emission in sound (403{mu}m at FWHM) than in X-ray (37{mu}m). Under the double pulse irradiation condition with the time delay at 0 ps and 4.6 ns, it was clearly observed that the acoustic signal intensity is enhanced in associated with X-ray intensity enhancements. The enhancements can be assigned to laser ablation dynamics such as pre-plasma formation and transient surface roughness formation induced by the pre-pulse irradiation. For the acoustic signal under the double-pulse irradiation with the time delay, there was a weak dependence observed on the pre-pulse irradiation position at the laser focus. It is consistent with a long breakdown filament formation which makes the microphone-detection less position-sensitive.
We demonstrate the high-efficiency generation of water-window soft x-ray emissions from polyethylene nanowire array targets irradiated by femtosecond laser pulses at the intensity of 4*10^19 W/cm^2. The experimental results indicate more than one order of magnitude enhancement of the water-window x-ray emissions from the nanowire array targets compared to the planar targets. The highest energy conversion efficiency from laser to water-window x-rays is measured as 0.5%/sr, which comes from the targets with the longest nanowires. Supported by particle-in-cell simulations and atomic kinetic codes, the physics that leads to the high conversion efficiency is discussed.
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